1. Field of the Invention
The present invention relates to circuits for electromagnetic coil-operated devices, and in particular for the control of drive coils in recorders for teleprinting devices.
2. Description of the Prior Art
In recording of information in readable form as well as in the plotting of information on carriers in the form of punched tapes, drive circuits including electromagnetic coils are widely known and utilized for momentary activation of individual recording elements such as a typeface or a tapepunching needle. Such devices are utilized in teleprinters which record information manually entered or received from a transmission line.
In the particular application of matrix printers, a particular matrix pattern is associated with each information character and selected needles in a corresponding needle matrix are individually activated by associated electromagnetic coils to move from a rest position into a printing position to mark or punch a hole in a moving paper tape. Each individual printing needle has a electromagnetic system associated with it which controls the movement of that particular printing needle against the writing surface. For the orderly and rapid operation of the printer, the electromagnetic system must embody a short activation time during which the coil is energized sufficiently to move the needle, a fast release time for returning the needle to a rest position, and moreover must have a low energy requirement.
An electromagnetic coil drive circuit suitable for use in such printers is known from German AS No. 2,515,124. In this arrangement, the coil of each electromagnet is in series with two switching transistors which are sequentially activated by two control pulses which have adjustable duty cycles. The circuit further requires the use of two separate voltage sources which are necessitated because the switching transistors are driven separately in time and act upon the magnetic coil first with a high voltage in order to quickly generate the necessary magnetic field to move the plunger, followed by supply of a second lower voltage to the coil as a maintenance current. The maintenance current exhibits essentially a current curve which is typical for a currentconstant circuit. Circuits of this type operate adequately, but require an undesired outlay because of the necessity of having two separate voltage sources. Moreover, the constant connection and disconnection of the voltage sources in such circuits leads to substantial heat losses and consequently a thermal load requiring a heat sink or other cooling devices.